The explosion of Krakatau in 1883 attracted a significant amount of attention, as discussed in my previous blog: The Explosive Roars of Krakatau. You would be forgiven, therefore, for believing that we’ve discovered all there is to know about one of the world’s most famous volcanoes and its most explosive eruption. However, there is still a lot that Krakatau – and Anak Krakatau or “Child of Krakatau” (Figure 1) – can tell us, not only about the 1883 eruption, but also about volcanoes related to subduction zones in general.

Figure 1: Anak Krakatau erupting.

The first thing to note is that there is still debate surrounding what triggered the 1883 eruption. It was originally thought that water entering the magma chamber had a role to play. Water is a magmatic volatile component (others including CO2, sulphur, chlorine, fluorine) that, when in contact with hot magma, instantly converts to steam. The sudden volume increase expands and fragments the magma, causing an explosive eruption. We term these eruptions phreatomagmatic. However, this process would mean that the tiny ash particles left behind in the rock record would be smaller than the grain-size we actually observe in the 1883 ashes, as observed on my fieldwork.

Another mechanism that has been proposed is magma mixing. This occurs when a new batch of magma enters the chamber, and the addition of pressure, heat, and volatiles can trigger an eruption. Although there is evidence for some magma mixing in the 1883 eruption of Krakatau, it is debated whether this amount would be enough to be deemed a significant factor. A recent study has also highlighted the role of the melting and incorporation of the rock surrounding the magma, which could have a similar effect. Again, it is debated whether this alone is enough to cause Krakatau to erupt so explosively.

The last process is ongoing constantly in the vast majority of volcanoes around the world. If this very common phenomenon is left ongoing for long enough, however, it too can lead to eruption. As the magma sits in the crust, it starts to cool and gradually crystallise. During this fractional crystallisation process, certain elements are preferentially removed from the melt to form the crystals, and this concentrates volatile components in the magma. These volatiles eventually become so concentrated they begin to come out of solution, forming a gas, which expands and fragments the magma. As before, this fragmentation causes explosive eruptions.

Krakatau has exhibited many different eruption styles, from the very explosive 1883 eruption to more recent gentle lava extrusion. Different eruptive styles present very different hazards to people. Whereas slow-moving lava flows are easy to avoid, hazards such as pyroclastic flows – associated with explosive eruptions – are deadlier. Volcanologists are therefore not only interested in whether and when an eruption will occur, but also what kind of eruption this will be. Krakatau’s eruption history makes it an ideal volcano to study to help us understand the processes underlying changes in eruptive style, and this is what my PhD has primarily been focussed on. Once the driving mechanisms of past eruptions are better understood, it is hoped that volcanologists who actively monitor volcanoes will be able to better recognise precursor signals to large explosive eruptions.